1. Field of the Invention
The present invention relates to an antenna device provided for a portable radio such as a portable cellular phone and, more particularly, to an antenna device which enables transmission and reception of signals at two different frequencies.
2. Description of the Related Art
In a portable radio device, such as a portable cellular phone, which is used while being carried outside, a whip antenna projecting from a housing of the radio is retracted into the housing so as to make the radio as compact as possible while the radio is being carried.
This type of portable radio uses a helical antenna constantly connected to feeding hardware to enable transmission and reception of a signal regardless of whether the whip antenna is housed in the housing or extended. Japanese Patent Publication of Translated Version (PCT) No. Hei-9-505956 discloses an antenna device which resonates at two radio different frequencies even while a whip antenna is retracted into the housing.
Referring now to FIG. 9, a prior art antenna device 100 comprises a fixed antenna section 130 and a slide antenna section 103. The fixed antenna section 130 includes a first helical antenna element 116 consisting of a first coil and a second helical antenna element 114 consisting of a second coil. Both antenna elements are helically wrapped around a cylindrical insulating sleeve 104. The coils are connected at one end to a conductive sleeve 109 attached to one end of the insulating sleeve 104. The conductive sleeve 109 is electrically connected to transmission and reception circuits of a portable radio (not shown), thus constituting means for attaching the antenna device 100 to the portable radio.
The slide antenna 103 is a rod antenna element disposed below an insulating cover 125. Slide antenna 103 passes through the insulating sleeve 104 and the conductive sleeve 109. The slide antenna 103 can be extended from and retracted into the portable radio in the axial direction.
When the slide antenna 103 is retracted to a housed position, the rod antenna element is positioned below the conductive sleeve 109. The rod antenna element is disconnected from the conductive sleeve 109 by means of the insulating cover 125 and hence is out of use. As shown in an equivalent circuit diagram shown in FIG. 10, the fixed antenna 130 is connected to the conductive sleeve 109, and the first and second helical antenna elements 116 and 114 resonate at resonance frequencies f11 and f12.
As shown in FIG. 11, when compared with the capability of an antenna device having one helical antenna which resonates at frequency f10 (denoted by a dashed line in FIG. 11), the bandwidth capability of the antenna device 100 covers a much wider range.
When the slide antenna 103 is extended, the lower end of the rod antenna element is electrically connected to the conductive sleeve 109, and the rod antenna acts as a whip antenna resonating at the frequency f10.
The conventional antenna device 100 enables transmission and reception of a signal regardless whether the slide antenna 103 is housed or extended. Further, as shown in FIG. 11, while the slide antenna 103 is housed, the antenna device 100 enables transmission and reception of a radio signal at two frequencies f11 and f12.
In recent years, a frequency band used for mobile communications equipment has differed from field to field. For example, an automobile telephone is allocated a range of 900 MHz, and a personal handyphone system (PHS) is allocated a range of 1.9 GHz. There is a desire for a portable radio capable of being used with both communications system. For this reason, there has been desired what is called a dual-band-capable antenna device which enables transmission and reception of a signal at either of the bands.
However, in the previously-described conventional antenna device 100, when the slide antenna section 103 is housed, the fixed antenna section 130 enables transmission and reception of a signal over a broad range. To this end, the two helical antenna elements are adjusted so as to resonate at comparatively close frequencies f11 and f12. Accordingly, the conventional antenna device 100 does not cope with communication at completely different frequency bands. If an attempt is made to cope with such communication through use of an antenna which is identical structure with the conventional antenna device 100, there is a need to connect the two helical antenna elements 114 and 116 having completely different extended strengths to the conductive sleeve 109 with a side-by-side configuration. This results in not only impairment of appearance of the portable radio but also an increase in the number of components, thereby rendering assembly of the portable radio laborious.
The conventional antenna device 100 causes only the rod antenna elements to serve as an antenna while the slide antenna section is extended. Accordingly, the antenna device 100 cannot enable the rod antenna elements to cope with two completely different frequency bands. Consequently, the conventional antenna device 100 cannot cause the antenna elements to resonate at two completely different frequencies and to enable transmission and reception of a signal at respective frequencies regardless of whether the slide antenna is housed or extended.